Unfortunately, Indagationes Mathematicae also has a different title, Proceedings of the Royal Dutch Academy of Sciences. So some libraries have it under "Koninklijke Nederlandse Akademie van Wetenschappen" instead of Indagationes. To make matters worse, the same volume of the journal will have different volume numbers under the two titles. This is one situation where the help of a competent librarian may save you a lot of frustration.